Biomedical Engineering Reference
In-Depth Information
(a)
(b)
FIGURE 2.5
Microstructure of carbons deposited in a fluidized bed. (a) A granular carbon with distinct growth
features. (b) An isotropic carbon without growth features. Both under polarized light. ×240. (From Bokros J.C.,
LaGrange L.D., and Schoen G.J. 1972.
Chem. Phys. Carbon
. 9:103-171. With permission.)
lower temperatures (<900°C), it is more likely that hydroxyl- or hydroxyapatite will form, while in a
dry atmosphere and at a higher temperature, β-whitlockite will be formed (Park and Lakes, 1992). Both
forms are very tissue-compatible and are used as bone substitutes in a granular form or a solid block.
The apatite form of calcium phosphate is considered to be closely related to the mineral phase of bone
and teeth.
The mineral part of bone and teeth is made of a crystalline form of calcium phosphate similar to
hydroxyapatite [Ca
10
(PO
4
)
6
(OH)
2
]. The apatite family of mineral [A
10
(BO
4
)
6
X
2
] crystallizes into hex-
agonal rhombic prisms and has unit cell dimensions
a
= 9.432 Å and
c
= 6.881 Å. The atomic structure
of hydroxyapatite projected down the
c
-axis onto the basal plane is shown in Figure 2.6. Note that the
hydroxyl ions lie on the corners of the projected basal plane and they occur at equidistant intervals
(3.44 Å) along the columns perpendicular to the basal plane and parallel to the
c
-axis. Six of the 10
calcium ions in the unit cell are associated with the hydroxyls in these columns, resulting in strong
interactions among them (Park and Lakes, 1992).
The ideal Ca:P ratio of hydroxyapatite is 10:6 and the calculated density is 3.219 g/cm
3
. Substitution
of OH with fluoride gives apatite greater chemical stability due to the closer coordination of fluoride
(symmetric shape) when compared to the hydroxyl (asymmetric, two atoms) by the nearest calcium.
This is why fluoridation of drinking water helps in resisting caries of the teeth (Park and Lakes, 1992).
The mechanical properties of synthetic calcium phosphates vary considerably (Table 2.10). The wide
variations in properties of polycrystalline calcium phosphates are due to the variations in the structure
